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In the film Dot and the Kangaroo, the Kangaroo warns Dot not to eat too much of the root that allows her to talk to animals because if she does she'll "know too much," which will make her "miserable."

If Godzilla absorbs too much radiation or if his body-temperature goes too high, he risks going into critical mass and could either explode and/or go into a meltdown. Either way, it's VERY bad news for the rest of the world.

Santa Claus -- The Movie (1985) has stardust as the phlebotinum in question: Adding it to their fodder gives Santa's reindeer their flying abilities. When elf inventor Patch journeys to the human world to prove his worth to Santa, he creates a lollipop that has a bit of stardust as an ingredient; whoever eats one can temporarily fly. They're hugely popular, and the Corrupt Corporate Executive who markets them demands that the follow-up should have more stardust added, so the effects will last longer; the villains subsequently learn that the resultant candy canes explode if kept near a heat source (i.e., a radiator) too long. This is because the stardust, which originated at the cold North Pole, becomes unstable when exposed to too much heat.

In Charlie and the Great Glass Elevator, Wonka gives the grandparents pills which de-age them exactly 20 years per pill—they all decide after taking the first one that they should split the whole bottle, which then made three of them babies and one of them negative two years old. Played for horror.

In one Fantasy Island a woman was given a potion to become beautiful for a time (an hour??) if she took one drop. Complications ensued and she was forcibly made to drink the whole bottle on the grounds that "if one drop is good the whole bottle is better!" She became a harridan.

The end of Planet of Spiders on Doctor Who-- the Great One, leader of the spiders, assembles a web of crystals that contain power--but when she completes it, the power destroys her. Also meant as a metaphor for the ego.

Those crystals are psychic super amplifiers so not just a metaphor, her ego *is* the power that destroys her

Older versions of Dungeons and Dragons has Potion Miscibility. That is if you mixed potions or drank a potion while under the effects of another potion you had to roll on a chart to see what would happen instead of the normal effects. A few of the effects are desirable, but possible effects include explosions and being poisoned.

Forgotten Realms had a few. Too many strong spells released at once tend to interact abnormally until they form one "spell-storm" on the scale up to Fantastic Nuke or overload and warp the Weave powering them. This way several civil wars among the Drow caused total destruction of their cities, supposedly including the creation of Great Rift—geographical feature about 170 miles long and 1000 feet deep. Myth Drannor had large extradimensional pockets built too densely—enough that they began to interfere, distort magic all around and compromised an originally impenetrable city-scale teleport denial ward—which becomes Very Bad News during an attack by a whole army of fiends.

Many cyberpunk games have some sort of psychological penalty for implanting too much cyberware, such as going insane or dying.

In The Witcher, you suffer poisoning when you drink potions. This prevents you from taking too many powerups at once. A justified trope since they are poisons, many of which are lethal to non-mutated humans, but not to the resistant Witchers.

Iron Man: Armored Adventures has this in the episode Best Served Cold. Whitney Stane has been over-using the Madame Masque image inducer, and the phlebotinum that powers it has started to affect her brain. Tony has to travel to the arctic to find the raw ore form of the material to help cure her but in the end ends up having to use the small ammount that powers his pacemaker.

People taking prescription drugs will often grow accustomed to the small amounts they take, and so they overdose when they take more and more to try to compensate. Unfortunately, the effective dose and the safe dose are both determined by different factors, and don't rise at the same rate when it comes to a lot of medications, so doing this means you're liable to, well, overdose. Better to switch up meds.